The ability to detect the “shift in the phase” of consecutive pulses of electromagnetic energy allows a Doppler radar to detect motion. The phase of the returning signal changes based upon the motion of targets with respect to the radar. A radar processor measures the phase change of the reflected pulses of energy and then converts that change to a velocity of the object, either toward or from the radar. Doppler processing requires pulse-to-pulse phase coherency over a series of many pulses to make this measurement.
Most radar designs co-locate the transmit and receive functions so they can share a timing reference to maintain coherency. In some radars, the transmit and receive functions are distributed functions. Successful radar ranging and motion detection typically requires the sharing of high bandwidth timing signals using coaxial cables which can be very sensitive to small variations and difficult to maintain.
Doppler radar generally has employed shared clocks in a transmitter and receiver in addition to a timing pulse, generally denoted as T0, which simultaneously triggers the transmission of a pulse and the start of radar echo signal collection. All pulse radars require precise knowledge of the transmit time, T0, and the target echo receive time to determine the target range. This requires that the T0 signal itself and the transmit and receive processes that follow must be accurate and repeatable to within 1 cycle of the shared reference clock. Any deviation from an identical interval degrades the ability to detect small phase shifts in the received echoes from pulse to pulse. For example, a 64 MHz shared clock this means a precision of better than 15.625 nanoseconds.
Clock jitter, i.e. timing variations caused by phase noise, on an analog-to-digital, or A/D, clock signal in a Doppler radar has a direct effect on the performance of the Doppler radar due to phase errors in the received signal. For example, a sampled 48 MHz intermediate frequency signal will have phase errors of 1 degree if the A/D sample jitter is as small as a variation of 58 picoseconds. The effect of clock jitter on a sample is to degrade the signal-to-noise ratio commonly denoted as SNR of the A/D converter. For many systems, clock jitter is the largest portion of the A/D SNR budget and if controlled, has a great influence on performance of a Doppler radar.
There is a need for removing clock jitter in synchronizing a receiver and transmitter for a radar installation. Traditionally, a timing signal of sufficient resolution must include a very steep onset of the timing pulse to render a very precise timing. Including many of the high harmonics is necessary to lend precision to the signal and therefore high bandwidth is necessary to convey a suitable timing signal. The high bandwidth requires either that the design of the radar collocates the transmit and receive functions or increases the system cost with a high bandwidth communication link between the transmitter and receiver functions.
Thus, there is an unmet need in the art for a synchronization system that removes the need for a timing signal with a very steep onset.